L7 2025 - Photosynthesis II PDF

1/20/20 HANDS-0N SUMMER-BREAK RESEARCH DOBBIE SMITH PRIZES University of Glasgow – Institute of Molecular, Cellular and Systems Biology and the Plant Science Research Group £1000 prize bursary...

1/20/20 HANDS-0N SUMMER-BREAK RESEARCH DOBBIE SMITH PRIZES University of Glasgow – Institute of Molecular, Cellular and Systems Biology and the Plant Science Research Group £1000 prize bursary and 4-8 weeks’ summer research experience Consult an approved member of the MCSB staff, agree a project, then … submit a letter of application to the Undergraduate School. Open to all first-year students in the Natural Sciences and allied studies at the University of Glasgow. Details available on the MOODLE site or from Prof. M. Blatt ([email protected]) The deadline for submissions is 12:00 noon Friday, 20.March 2020 1 Photosynthesis 2 – dark reactions What are the ‘dark’ reactions? How is carbon captured and fixed into sugar? What is RuBisCO? What is photorespiration? What is C4 photosynthesis? Biology, Campbell&Reese 7th ed., Chap 10 2 1  1/20/20 How were dark reactions identified? Fast CO2 injection Rapid ‘quench’ (perchloric acid) to stop reactions 2D chromatography improves separation of intermediates 3 How were dark reactions identified? Fast CO2 injection Rapid ‘quench’ (perchloric acid) to stop reactions 14CO 2 Chlorella Injection suspension Port Perchloric Acid Light Source & Heat Filters 4 2  1/20/20 14CO pulse-chase analysis of the C3 cycle 2 2D chromatography improves separation of intermediates 5 s labelling Time sequence of product 14C labelling indicates sequence of transitions 30 s labelling 5 Photosynthesis divides between ‘light’ and ‘dark’ reactions Calvin-Benson cycle identified in 1948 – led to Nobel prize in Chemistry Time sequence of product 14C labelling indicates sequence of transitions Key enzyme is RuBisCO (Ribulose-bisphosphate- caboxylase/oxygenase) 6 3  1/20/20 Light and Dark reactions Gaseous CO2 and the 5-carbon sugar ribulose 1,5-bisphosphate form two molecules of 3-phosphoglycerate (PGA) Reaction is metabolically irreversible RuBisCO makes up about 50% of the soluble protein in plant leaves, and is the most abundant enzymes in nature RuBisCO = 8 large subunits + 8 small subunits Large subunits nuclear-encoded; small subunits chloroplast-encoded 7 Chlorophyll capture light RuBisCO also use O2 to catalyze a competing oxygenation reaction Normally carboxylation is 3 times greater than oxygenation Photorespiration recycles the (toxic) products of the oxygenation reaction; it consumes NADH, ATP to give glyoxylate, serine, glycine and CO2 8 4  1/20/20 Alternative Photosynthetic Strategies Many plants, algae and photosynthetic bacteria (cyanobacteria) have evolved methods to get around the limitations of RuBisCO Cyanobacteria form Carboxysomes to concentrate CO2 around RuBisCO Some algae form Pyrenoids to concentrate CO2 around RuBisCO Some Angiosperms have evolved C4 photosythesis to ’feed’ RuBisCO Some Angiosperms have evolved CAM photosythesis to ’feed’ RuBisCO 9 Carboxysomes and Pyrenoids Cyanobacteria form Carboxysomes to concentrate CO2 around RuBisCO Carboxysomes are semi-crystalline protein structures incorporating RuBisCO Cyanobacteria express transport proteins to accumulate CO2 and HCO3- inside Pyrenoids are membrane-bound structures found in the chloroplasts of some algae Pyrenoids, like carboxysomes, include RuBisCO and concentrate CO2 10 5  1/20/20 C4 Photosynthesis C4 photosythesis evolved over 68 times in angiosperms (also known as the Hatch-Slack pathway) C4 photosythesis fixes HCO3- with pyruvate into malate, then transport malate to a second cell type for decarboxylation and refixation by RuBisCO Vein Vein Discovered in 1950’s 1 HCO3- CO2 Pyr 5 PEP PEP 2 Pyr RuBisCO OAA C4 photosynthesis is more efficient CO2 3 Pyr Mal Mal OAA 4 because it fixes HCO3- first (avoids O2) Bundle Sheath Mesophyll 11 C4 Photosynthesis Mesophyll Cell Rice Maize Bundle Sheath Cell Vein Vein C4 photosythesis depends on spatial separation of the C4 and C3 (RuBisCO) fixation reactions 1 HCO3- CO2 Pyr 5 PEP PEP The only major C4 crop is maize 2 Pyr RuBisCO OAA CO2 3 Pyr Mal Mal OAA 4 Bundle Sheath Mesophyll 12 6  1/20/20 CAM Photosynthesis CAM = Crassulacean Acid Metabolism CAM photosythesis depends on temporal separation of the C4 and C3 (RuBisCO) fixation reactions CAM plants store C4 acids (malate) in the vacuole for decarboxylation and refixation by RuBisCO in the day 13 CAM Photosynthesis CAM requires fine control of gene expression over the day-night cycle CAM uses an ‘inverted’ cycle of stomatal opening/closing to save water and concentrate CO2 in the leaf 'Inverted’ stomatal diel cycle 14 7  1/20/20 CAM Photosynthesis Like C4 photosynthesis, CAM has evolved multiple times across the angiosperms The only major CAM crop is pineapple 15 CO2/O2 limits the efficiency of the C3 cycle In C3 plants photorespiration accounts for ~30-50% of RuBisCO turnover CAM achieves much more savings in water use 16 8  1/20/20 Is CAM better for engineering? CAM will require engineering of approximately 35-40 gene targets with their promoters 17 Summary Dark reactions use NADPH and ATP with RuBisCO to fix CO2 and regenerate RuBP C3 photosythesis depends on RuBisCO selectivity between CO2 and O2 C4 and CAM photosynthesis get around the CO2/O2 limitation by introducing a ‘pre-fixation’ step to capture C as a 4-carbon acid C4 and CAM have a metabolic cost, but they are well-suited to growth under high light (extra energy) and reduced water availability 18 9